Autoclave AKA: Pressure Cooker

Fig 1: Autoclave with integral heater

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Summary

A device used to sterilise equipment and nutrient solutions.

Operation

Autoclaves are used in schools to sterilise equipment for microbiological work and many varieties are available to suit the needs of your school. Some varieties resemble (and essentially are) domestic pressure cookers in that they need to be heated from below on a hotplate or gas ring. Other more expensive varieties may contain a heating element which allows the unit to achieve a desired temperature and pressure.

When buying an autoclave, several factors need to be taken into account. A gas supply will be needed if you choose an autoclave without an integral heating element. The physical dimensions of the unit may be an issue if large equipment is to be sterilised or if you intend to sterilise a large volume of smaller equipment. You also need to know if the temperature range of the device will be suitable for your needs.

Because the cost of an autoclave can be relatively high, it is recommended that you do your homework before you purchase one. Work out how much and what kind of microbiology you intend to conduct in the future, what budget range you have available and what space restrictions you may have.

Domestic pressure cookers are fine for small scale school use and are relatively inexpensive. Large autoclaves may be able to sterilise more equipment in one go but some take a long time to heat up and cool down. Lab suppliers should be able to let you see the full product specifications including temperature ranges, physical size and warm-up times before you decide which is best for your school.

Although the design and internal workings of autoclave vary, they essentially perform the same role. Under normal atmospheric pressure, water boils at approximately 100°C. When water is heated in a sealed autoclave, it is possible to heat liquid water to a much higher temperature. As the water is heated, the pressure rises due to the constant volume of the container. The boiling point of the water is raised because the amount of energy needed to form steam against the higher pressure is increased.

Self heating automatic autoclaves may have differing temperature and cycle times. Cycle times refer to the level of temperature for a specific length of time. For example an autoclave may have a cycle of 126°C for 11 minutes. This may be too higher temperature for sterilisation of certain types of agar. You should be able to obtain nutrient agar specifications from suppliers, showing the temperature ranges and recommended sterilisation times.

Sterilisation efficiency tape can be used which shows if a good sterilisation has been achieved. Some are simple white tapes which change to black after exposure to steam sterilisation, some are more detailed labels which have panels which turn blue at a specific temperature. It is important to check regularly that your autoclave or pressure cooker is working correctly by using these more detailed labels. They are available in different temperature ranges for a variety of applications.

Because autoclaves and domestic pressure cookers vary significantly in design and operation, read the instructions that are supplied with the unit carefully before use. Check valves are not blocked and rubber gaskets are intact as per the manufacturer's guidelines.

Pressure Systems Safety Regulations 2000 state that all pressure vessels have to be regularly checked by a competent person. Refer to CLEAPSS or SSERC publications for more details. Ensure that your autoclave is being checked regularly as corrosion or damage to the safety valves could result in potentially hazardous results.

Safety

The contents of this page are for information only. Please refer to CLEAPSS, SSERC or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.